Del Potro required four match points - two on Zverev’s serve in the ninth game, two on his own serve in the 10th game - to win this first encounter against the German.

Del Potro, who lost his serve for the only time in the match in the second game, won nine of his 15 service games at love.

Zverev, in the hunt for a sixth ATP title of the season, lost his composure after netting a backhand to surrender his serve in the fifth game of the third set to the Argentine. He repeatedly smashed his racket until it was a mangled mess.

“For me, it’s impossible to lose a match when I’ve only been broken one time,” Zverev said. “The game where he broke me I was up 40-15 and I was just unfocused there.”

Del Potro knew Zverev’s reaction to losing serve was likely a good omen.

“I saw his frustration, for sure,” del Potro said. “He’s still very young and has to keep learning. He has everything to be in the top of the game.”

Del Potro will John Isner of the U.S. or Viktor Troicki of Serbia in the last eight on Friday.